Ross was born in Watertown, Carver County, Minnesota, the daughter of Gordon and Ellen (Hamilton) Ross,[1] She lived in Waconia, then moved to Willmar, and eventually to Albert Lea, Minnesota.[2] At the age of 13, she changed the spelling of her name from "Marian" to "Marion" because she thought it would look better on a marquee.

Ross enrolled in San Diego State University,[5] where she was named the school's most outstanding actress. After graduation in 1950, she performed in summer theater in La Jolla, California. The director was impressed by her talent and recommended that she try for work in films.

Ross' best known role is on the sitcom Happy Days, which aired for 11 seasons on ABC, from 1974 to 1984. She portrayed matriarch Marion Cunningham, mother of Richie, Joanie, and (briefly) Chuck. She received Primetime Emmy Award nominations for her work on the show in 1979 and 1984. Ross later reprised Marion Cunningham on the spin-off series Joanie Loves Chachi and on Family Guy.

Between 1978 and 1986, she appeared as different characters on The Love Boat. In the 1986–87 television season, Ross became a series regular, playing Emily Haywood. She later starred in the short-lived, critically acclaimed comedy-drama Brooklyn Bridge, which ran on CBS from 1991 to 1993. The series won a Golden Globe Award and was nominated for a Primetime Emmy Award following its first season.

On April 19, 2011, Ross and four of her Happy Days co-stars, Erin Moran, Don Most, Anson Williams, and the estate of Tom Bosley filed a $10 million breach-of-contract lawsuit against CBS, which owns the show, claiming they had not been paid for merchandising revenue owed under their contracts. The cast members claimed they had not received revenue from show-related items, including comic books, T-shirts, scrapbooks, trading cards, games, lunch boxes, dolls, toy cars, magnets, greeting cards, and DVDs where their images appear on the box covers. Under their contracts, they were supposed to be paid five percent from the net proceeds of merchandising if their sole images were used, and half that amount if they were in a group. CBS said it owed the actors $8,500 and $9,000 each, most of it from slot machine revenue, but the group said they were owed millions. The lawsuit was initiated after Ross was informed by a friend playing slots at a casino of a "Happy Days" machine on which players win the jackpot when five Marion Rosses are rolled.[10]

In October 2011, a judge rejected the group's fraud claim, which rejects their claim to millions of dollars in potential damages.[11] On June 5, 2012, a judge denied a motion filed by CBS to have the case thrown out, which meant it would go to trial on July 17 if the matter was not settled by then.[12] In July 2012, the actors settled their lawsuit with CBS. Each received a payment of $65,000 and a promise by CBS to continue honoring the terms of their contracts.[13][14]